Effective stack sizes determine what kinds of hands you can play profitably (effective stack sizes means that if I have 300 bb's and you have 100bb's, we're only playing for 100bb's if we get heads up - the rest of my stack is meaningless headsup against your stack). Shortstacks (20bb's) are looking for top pair and overpair type hands (AK, AQ, AJ, KK, QQ, JJ, TT, etc.). At around 70to 120bb's, you're looking for hands that you can take past the flop - this would include more suited connectors and connected cards. At 180bb's+, you can play a lot of speculative hands.
The reason the stack size is important is because of the limits effective stacks place on postflop play. If you only have 20bb's, once you get to the flop or the turn your options are pretty much shove or fold. If you have 100bb's, you can make a pot sized bet on the flop, turn, and river in a raised pot. More options are available when you get deepstacked, say 200bb's deep.
That said, if the max buyin is 100bb's, effective stacks for you when you first buy in is at most 100bb's. I personally dislike playing w shortstacks, cuz it turns into a shove fest preflop or on the flop, so I avoid tables w ss's. Deepstacked players have a license to play a lot of speculative hands, so they see a lot of flops - I prefer having them on my right, but it's not a requirement for me, they end up playing a lot of trashy hands that they have to fold to aggression (but be careful when they fight back, they may have a very strong hand that's extremely well concealed).
Don't overthink this though - at 2nl and 5nl, most players aren't all that aware of how stack sizes impact the kinds of hands they should be playing.